📜 33 JavaScript concepts every developer should know.
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Repo: leonardomso/33-js-concepts
What's inside
This repository helps developers master core JavaScript concepts. Each concept includes clear explanations, practical code examples, and curated resources.
Start learning at 33jsconcepts.com →
Primitive Types
Learn JavaScript's 7 primitive types: string, number, bigint, boolean, undefined, null, and symbol. Understand immutability, typeof quirks, and autoboxing.
Primitives vs Objects
Learn how JavaScript primitives and objects differ in behavior. Understand immutability, call-by-sharing semantics, why mutation works but reassignment doesn't, and how V8 actually stores values.
Type Coercion
Learn JavaScript type coercion and implicit conversion. Understand how values convert to strings, numbers, and booleans, the 8 falsy values, and how to avoid common coercion bugs.
Equality Operators
Learn JavaScript equality operators == vs ===, typeof quirks, and Object.is(). Understand type coercion, why NaN !== NaN, and why typeof null returns 'object'.
Scope and Closures
Learn JavaScript scope and closures. Understand the three types of scope, var vs let vs const, lexical scoping, the scope chain, and closure patterns for data privacy.
Call Stack
Learn how the JavaScript call stack tracks function execution. Understand stack frames, LIFO ordering, execution contexts, stack overflow errors, and debugging with stack traces.
Event Loop
Learn how the JavaScript event loop handles async code. Understand the call stack, task queue, microtasks, and why Promises always run before setTimeout().
IIFE, Modules & Namespaces
Learn how to organize JavaScript code with IIFEs, namespaces, and ES6 modules. Understand private scope, exports, dynamic imports, and common module mistakes.
DOM
Learn how the DOM works in JavaScript. Understand how browsers represent HTML as a tree, select and manipulate elements, traverse nodes, and optimize rendering performance.
Fetch API
Learn how to make HTTP requests with the JavaScript Fetch API. Understand GET, POST, response handling, JSON parsing, error patterns, and AbortController for cancellation.
Web Workers
Learn Web Workers in JavaScript for running code in background threads. Understand postMessage, Dedicated and Shared Workers, and transferable objects.
Factories and Classes
Learn JavaScript factory functions and ES6 classes. Understand constructors, prototypes, private fields, inheritance, and when to use each pattern.
this, call, apply, bind
Learn how JavaScript's 'this' keyword works and how to control context binding. Understand the 5 binding rules, call/apply/bind methods, arrow functions, and common pitfalls.
Object Creation & Prototypes
Learn JavaScript's prototype chain and object creation. Understand how inheritance works, the new operator's 4 steps, Object.create(), Object.assign(), and prototype methods.
Inheritance & Polymorphism
Learn inheritance and polymorphism in JavaScript — extending classes, prototype chains, method overriding, and code reuse patterns.
Callbacks
Learn JavaScript callbacks, functions passed to other functions to be called later. Understand sync vs async callbacks, error-first patterns, callback hell, and why Promises were invented.
Promises
Learn JavaScript Promises for handling async operations. Understand how to create, chain, and combine Promises, handle errors properly, and avoid common pitfalls.
async/await
Learn async/await in JavaScript. Syntactic sugar over Promises that makes async code readable. Covers error handling with try/catch, parallel execution with Promise.all, and common pitfalls.
Generators & Iterators
Learn JavaScript generators and iterators. Understand yield, the iteration protocol, lazy evaluation, infinite sequences, and async generators with for await...of.
Higher-Order Functions
Learn higher-order functions in JavaScript. Understand functions that accept or return other functions, create reusable abstractions, and write cleaner code.
Pure Functions
Learn pure functions in JavaScript. Understand the two rules of purity, avoid side effects, and write testable, predictable code with immutable patterns.
map, reduce, filter
Learn map, reduce, and filter in JavaScript. Transform, filter, and combine arrays without mutation. Includes method chaining and common pitfalls.
Recursion
Learn recursion in JavaScript. Understand base cases, recursive calls, the call stack, and patterns like factorial, tree traversal, and memoization.
Currying & Composition
Learn currying and function composition in JavaScript. Build reusable functions from simple pieces using curry, compose, and pipe for cleaner, modular code.
JavaScript Engines
Learn how JavaScript engines work. Understand V8's architecture, parsing, compilation, JIT optimization, hidden classes, inline caching, and garbage collection.
Error Handling
Learn JavaScript error handling with try/catch/finally. Understand Error types, custom errors, async error patterns, and best practices for robust code.
Regular Expressions
Learn regular expressions in JavaScript. Covers pattern syntax, character classes, quantifiers, flags, capturing groups, and methods like test, match, and replace.
Modern JS Syntax
Learn modern JavaScript ES6+ syntax. Covers destructuring, spread/rest operators, arrow functions, optional chaining, nullish coalescing, and template literals.
ES Modules
Learn ES Modules in JavaScript. Understand import/export syntax, why ESM beats CommonJS, live bindings, dynamic imports, top-level await, and how modules enable tree-shaking.
Data Structures
Learn JavaScript data structures from built-in Arrays, Objects, Maps, and Sets to implementing Stacks, Queues, and Linked Lists. Understand when to use each structure.
Algorithms & Big O
Learn Big O notation and algorithms in JavaScript. Understand time complexity, implement searching and sorting algorithms, and recognize common interview patterns.
Design Patterns
Learn JavaScript design patterns like Module, Singleton, Observer, Factory, Proxy, and Decorator. Understand when to use each pattern and avoid common pitfalls.
Clean Code
Learn clean code principles for JavaScript. Covers meaningful naming, small functions, DRY, avoiding side effects, and best practices to write maintainable code.
Ready to go deeper? These advanced topics build on the fundamentals above.
Hoisting
Learn how JavaScript hoists variable and function declarations. Understand why var behaves differently from let and const, function hoisting order, and how to avoid common bugs.
Temporal Dead Zone
Learn the Temporal Dead Zone (TDZ) in JavaScript. Understand why accessing let and const before declaration throws errors, and how TDZ differs from var hoisting.
Strict Mode
Learn JavaScript strict mode and how 'use strict' catches common mistakes. Understand silent errors it prevents, forbidden syntax, and when to use it.
Property Descriptors
Learn JavaScript property descriptors. Understand writable, enumerable, and configurable attributes, Object.defineProperty(), and how to create immutable object properties.
Getters & Setters
Learn JavaScript getters and setters. Understand how to define computed properties with get and set, validate data on assignment, and create reactive object behavior.
Object Methods
Learn essential JavaScript Object methods. Master Object.keys(), Object.values(), Object.entries(), Object.fromEntries(), Object.freeze(), Object.seal(), and object cloning patterns.
Proxy & Reflect
Learn JavaScript Proxy and Reflect APIs. Understand how to intercept object operations, create reactive systems, implement validation, and build powerful metaprogramming patterns.
WeakMap & WeakSet
Learn JavaScript WeakMap and WeakSet. Understand weak references, automatic garbage collection, private data patterns, and when to use them over Map and Set.
Memory Management
Learn JavaScript memory management. Understand the memory lifecycle, stack vs heap allocation, memory leaks, and how to profile memory usage in DevTools.
Garbage Collection
Learn how JavaScript garbage collection works. Understand mark-and-sweep, reference counting, generational GC, and how to write memory-efficient code.
Debouncing & Throttling
Learn debouncing and throttling in JavaScript. Understand how to optimize event handlers, reduce API calls, improve scroll performance, and implement both patterns from scratch.
Memoization
Learn memoization in JavaScript. Understand how to cache function results, optimize expensive computations, implement memoization patterns, and when caching hurts performance.
Tagged Template Literals
Learn JavaScript tagged template literals. Understand how to create custom string processing functions, build DSLs, sanitize HTML, and use popular libraries like styled-components.
Computed Property Names
Learn JavaScript computed property names. Understand how to use dynamic keys in object literals, create objects from variables, and leverage Symbol keys.
localStorage & sessionStorage
Learn Web Storage APIs in JavaScript. Understand localStorage vs sessionStorage, storage limits, JSON serialization, storage events, and security considerations.
IndexedDB
Learn IndexedDB for client-side storage in JavaScript. Understand how to store large amounts of structured data, create indexes, perform transactions, and handle versioning.
Cookies
Learn JavaScript cookies. Understand how to read, write, and delete cookies, cookie attributes like HttpOnly and SameSite, security best practices, and when to use cookies vs Web Storage.
Event Bubbling & Capturing
Learn JavaScript event bubbling and capturing. Understand the three phases of event propagation, stopPropagation(), event flow direction, and when to use each phase.
Event Delegation
Learn event delegation in JavaScript. Understand how to handle events efficiently using bubbling, manage dynamic elements, reduce memory usage, and implement common delegation patterns.
Custom Events
Learn JavaScript custom events. Understand how to create, dispatch, and listen for CustomEvent, pass data between components, and build decoupled event-driven architectures.
Intersection Observer
Learn the Intersection Observer API. Understand how to detect element visibility, implement lazy loading, infinite scroll, and animate elements on scroll efficiently.
Mutation Observer
Learn the Mutation Observer API. Understand how to watch DOM changes, detect attribute modifications, observe child elements, and replace deprecated mutation events.
Resize Observer
Learn the Resize Observer API. Understand how to respond to element size changes, build responsive components, and replace inefficient window resize listeners.
Performance Observer
Learn the Performance Observer API. Understand how to measure page performance, track Long Tasks, monitor layout shifts, and collect Core Web Vitals metrics.
JSON Deep Dive
Learn advanced JSON in JavaScript. Understand JSON.stringify() replacers, JSON.parse() revivers, handling circular references, BigInt serialization, and custom toJSON methods.
Typed Arrays & ArrayBuffers
Learn JavaScript Typed Arrays and ArrayBuffers. Understand binary data handling, DataView, working with WebGL, file processing, and network protocol implementation.
Blob & File API
Learn JavaScript Blob and File APIs. Understand how to create, read, and manipulate binary data, handle file uploads, generate downloads, and work with FileReader.
requestAnimationFrame
Learn requestAnimationFrame in JavaScript. Understand how to create smooth 60fps animations, sync with browser repaint cycles, and optimize animation performance.
This project has been translated into 40+ languages by our amazing community!
We welcome contributions! See our Contributing Guidelines for details.
MIT © Leonardo Maldonado
.claude/
CLAUDE.md
skills/
concept-workflow/
SKILL.md
fact-check/
SKILL.md
resource-curator/
SKILL.md
seo-review/
SKILL.md
test-writer/
SKILL.md
write-concept/
SKILL.md
.DS_Store
.github/
FUNDING.yml
workflows/
tests.yml
.gitignore
.opencode/
skill/
concept-workflow/
SKILL.md
fact-check/
SKILL.md
resource-curator/
SKILL.md
seo-review/
SKILL.md
test-writer/
SKILL.md
write-concept/
SKILL.md
CODE_OF_CONDUCT.md
CONTRIBUTING.md
docs/
5c8wamucvfketshf1eyrw254gz94jwre.txt
beyond/
concepts/
blob-file-api.mdx
computed-property-names.mdx
cookies.mdx
custom-events.mdx
debouncing-throttling.mdx
event-bubbling-capturing.mdx
event-delegation.mdx
garbage-collection.mdx
getters-setters.mdx
hoisting.mdx
indexeddb.mdx
intersection-observer.mdx
javascript-type-nuances.mdx
json-deep-dive.mdx
localstorage-sessionstorage.mdx
memoization.mdx
memory-management.mdx
mutation-observer.mdx
object-methods.mdx
performance-observer.mdx
property-descriptors.mdx
proxy-reflect.mdx
requestanimationframe.mdx
resize-observer.mdx
strict-mode.mdx
tagged-template-literals.mdx
temporal-dead-zone.mdx
typed-arrays-arraybuffers.mdx
weakmap-weakset.mdx
getting-started/
overview.mdx
concepts/
algorithms-big-o.mdx
async-await.mdx
call-stack.mdx
callbacks.mdx
clean-code.mdx
currying-composition.mdx
data-structures.mdx
design-patterns.mdx
dom.mdx
equality-operators.mdx
error-handling.mdx
es-modules.mdx
event-loop.mdx
factories-classes.mdx
generators-iterators.mdx
higher-order-functions.mdx
http-fetch.mdx
iife-modules.mdx
inheritance-polymorphism.mdx
javascript-engines.mdx
map-reduce-filter.mdx
modern-js-syntax.mdx
object-creation-prototypes.mdx
primitive-types.mdx
primitives-objects.mdx
promises.mdx
pure-functions.mdx
recursion.mdx
regular-expressions.mdx
scope-and-closures.mdx
this-call-apply-bind.mdx
type-coercion.mdx
web-workers.mdx
contributing.mdx
docs.json
favicon.ico
getting-started/
about.mdx
how-to-learn.mdx
learning-paths.mdx
prerequisites.mdx
index.mdx
og-image.png
robots.txt
schema-inject.js
translations.mdx
github-image.png
index.js
LICENSE
opencode.jsonc
package-lock.json
package.json
README.md
tests/
advanced-topics/
algorithms-big-o/
algorithms-big-o.test.js
data-structures/
data-structures.test.js
design-patterns/
design-patterns.test.js
error-handling/
error-handling.test.js
es-modules/
es-modules.test.js
modern-js-syntax/
modern-js-syntax.test.js
regular-expressions/
regular-expressions.test.js
async-javascript/
callbacks/
callbacks.dom.test.js
callbacks.test.js
beyond/
browser-storage/
cookies/
cookies.dom.test.js
cookies.test.js
indexeddb/
indexeddb.test.js
localstorage-sessionstorage/
localstorage-sessionstorage.dom.test.js
localstorage-sessionstorage.test.js
data-handling/
blob-file-api/
blob-file-api.dom.test.js
blob-file-api.test.js
json-deep-dive/
json-deep-dive.test.js
requestanimationframe/
requestanimationframe.test.js
typed-arrays-arraybuffers/
typed-arrays-arraybuffers.test.js
events/
custom-events/
custom-events.dom.test.js
custom-events.test.js
event-bubbling-capturing/
event-bubbling-capturing.dom.test.js
event-delegation/
event-delegation.test.js
language-mechanics/
hoisting/
hoisting.test.js
strict-mode/
strict-mode.test.js
temporal-dead-zone/
temporal-dead-zone.test.js
memory-performance/
debouncing-throttling/
debouncing-throttling.test.js
garbage-collection/
garbage-collection.test.js
memoization/
memoization.test.js
memory-management/
memory-management.test.js
modern-syntax-operators/
computed-property-names/
computed-property-names.test.js
tagged-template-literals/
tagged-template-literals.test.js
objects-properties/
getters-setters/
getters-setters.test.js
object-methods/
object-methods.test.js
property-descriptors/
property-descriptors.test.js
proxy-reflect/
proxy-reflect.test.js
weakmap-weakset/
weakmap-weakset.test.js
observer-apis/
intersection-observer/
intersection-observer.dom.test.js
intersection-observer.test.js
mutation-observer/
mutation-observer.dom.test.js
performance-observer/
performance-observer.test.js
resize-observer/
resize-observer.test.js
type-system/
javascript-type-nuances/
javascript-type-nuances.test.js
functional-programming/
currying-composition/
currying-composition.test.js
higher-order-functions/
higher-order-functions.test.js
map-reduce-filter/
map-reduce-filter.test.js
pure-functions/
pure-functions.test.js
recursion/
recursion.test.js
functions-execution/
async-await/
async-await.test.js
event-loop/
event-loop.test.js
generators-iterators/
generators-iterators.test.js
iife-modules/
iife-modules.test.js
promises/
promises.test.js
fundamentals/
call-stack/
call-stack.test.js
equality-operators/
equality-operators.test.js
javascript-engines/
javascript-engines.test.js
primitive-types/
primitive-types.test.js
primitives-objects/
primitives-objects.test.js
scope-and-closures/
scope-and-closures.test.js
type-coercion/
type-coercion.test.js
object-oriented/
factories-classes/
factories-classes.test.js
inheritance-polymorphism/
inheritance-polymorphism.test.js
object-creation-prototypes/
object-creation-prototypes.test.js
this-call-apply-bind/
this-call-apply-bind.test.js
web-platform/
dom/
dom.test.js
http-fetch/
http-fetch.test.js
TRANSLATIONS.md
vitest.config.js265 Rust rules your AI coding agent can use to write better code. Current for Rust 1.96 (2024 edition). Works with Claude Code, Cursor, Windsurf, Copilot, Codex, Aider, Zed, Amp, Cline, and pretty much any other agent that supports skills.
@leonardomsoFAQ
33-js-concepts is a Claude Code plugin with 6 hand-picked skills for content work, indexed on Flowy. Install it with the command on its page. It includes concept-workflow, fact-check, resource-curator. Its skills do not fire on their own yet. Request auto-invocation to have Flowy route them as you prompt. Free and open source.